In patent document 1 listed below, there is disclosed a flat two-chamber side airbag having a configuration where the neighborhoods of both side edges of a long and narrow rectangular partition wall (tether) are sewn to two base cloths (panels) and where the two panels are superimposed on top of each other and the neighborhoods of outer peripheral edge portions of the two panels are sewn together in a state in which the partition wall has been folded in two upward. This side airbag is stored in the side portion of a seat in a vehicle and protects the waist and the chest of an occupant by inflating and deploying between the occupant and the vehicle body side portion during a side impact or the like.
However, in the side airbag described above, the neighborhoods of the outer peripheral edge portions of the two panels are superimposed on top of each other and sewn together, so in order to make the thickness dimension (bag thickness) of the side airbag in the inflated and deployed state thicker to increase impact absorption, it is necessary to make the width dimensions (length dimensions along the vehicle body front-and-rear direction in the inflated and deployed state) of the two panels longer. For that reason, there is the potential for the side airbag to increase in size and for the time when the side airbag completely deploys to become later.
In patent document 2 listed below, there is disclosed a flat box-shaped side airbag (a so-called two-chamber 3D side airbag) comprising: two panels (main panels) having substantially the same shape that are placed facing each other; one belt-shaped panel (side panel) that is sewn to outer peripheral edge portions of the two main panels to thereby form a bag body together with the two main panels; and a tether that is formed in a long and narrow rectangular shape, is disposed inside the bag body, and is sewn to the two main panels to thereby section the inside of the bag body into upper and low chambers. In this two-chamber 3D side airbag, the bag thickness can be made thicker as a result of the side panel being set, so impact absorption can be ensured and an undue increase in size can be avoided.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2004-338698    Patent Document 2: JP-A No. 2007-38712